This invention in general relates to the extraction of signals in an electronic sensor and, more particularly, to a method and apparatus that uses a frequency based technique for the extraction of signals in electronic sensors such as gyroscopes.
Electronic sensors manufactured by MEMS technology are playing key roles in many areas. For instance, micro mechanical gyroscopes have enabled several important control systems in transportation and commercial applications. Other microdevices such as pressure sensors, accelerometers, actuators, and resonators fabricated by MEMS technology are also used in many areas.
In the area of micro gyroscopes, there is a need to provide improve techniques to extract components of interest from signals such as a signal component that is indicative of an angular rate that was externally induced to the gyroscope. One type of micro gyroscope contains two movable proof masses. The proof masses are vibrated in the same plane (in-plane) at a predetermined frequency by a motor in the gyroscope. The motor may include electrodes that drive the proof masses in the same plane in an oscillatory manner. The oscillation of the proof masses is controlled to a frequency near the resonant frequency of the proof masses.
In addition to a set of proof masses and drive electrodes, the gyroscope also contains sensing electrodes around the proof masses that report signals indicative of the movement of each proof mass. In particular, certain electrodes sense the in-plane movement of the proof masses. Other electrodes sense the out-of-plane movement of the proof masses. With appropriate signal processing and extraction circuitry, an angular rate component can be recovered from the reported signal of the electrodes sensing the out-of-plane movement of the proof masses.
A variety of techniques have been applied to extract a signal of interest in a gyroscope. These techniques, however, are limited in accuracy, reliability, and cost. In particular, the angular rate component of a signal from the out-of-plane electrodes must be isolated and extracted from several extraneous components such as the motor drive feedthrough, the quadrature component, the resonance of the motor drive feedthrough, and other system resonance and noise. Some of these extraneous components can be greater than the angular rate component. Moreover, the angular rate component of the signal varies considerably in magnitude and frequency over a full operating range of the gyroscope. There is also a variation from device to device that affects the relationship of the angular rate component to other components in the signal.
Current schemes to isolate and extract a signal from the gyroscope use a dual windowing scheme to extract the angular rate externally induced to the device. For instance, one technique known for extracting a reported angular rate signal 22 from a gyroscope element 20 is shown in FIG. 1. In this technique, two signals 24, 26 are generated from the gyroscope element 20. The first signal 24 is reported from electrodes that are in the same plane as the proof masses (in-plane electrodes). The first signal 24 is indicative of the oscillation of the gyroscope moving in an in-plane motion. One use of the first signal 24 is for motor drive control circuitry 28 to provide a control loop that maintains the oscillation of the proof masses to a frequency near the resonant frequency of the proof masses. The second signal 26 is reported from the electrodes that are not in the same plane as the proof masses (out-of-plane electrodes). The second signal 26 contains a signal component that is representative of the angular rate that is being externally induced on the gyroscope element 26. The second signal 26, however, also contains other extraneous signal components.
In this case, the signal processing circuitry includes a bandpass filter 30 that receives the second signal 26 and allows certain signal components that fall within a selected range of frequencies to pass through the filter. The output of the bandpass filter 30 is a complex filtered second signal 32, in the time domain, that contains an angular rate component and a quadrature component. The angular rate component of the complex filtered second signal 32 is one of the signal components of interest of the gyroscope. The quadrature component of the complex filtered second signal 32 is an error caused by the drive force of the gyroscope when it oscillates out-of-plane in an elliptical manner. The angular rate component and the quadrature component are offset by ninety degrees.
The system here uses a dual windowing scheme that includes the generation of two windows. The two windows are generated by a phase locked loop 34. The windows are set at ninety-degrees out of phase from each other in order to capture the two signal components. In particular, the quadrature component can be extracted by inputting the complex filtered second signal 32 to a first multiplier 36. The first multiplier 36 demodulates the complex filtered second signal 32 by multiplying the complex filtered signal 32 by a reference signal 38 that is a function of the first signal 24. The reference signal 38 is essentially a reference sinusoid that includes the in-plane signal amplitude and the resonant frequency of the proof masses. The reference signal 38 is generated from the phase locked loop 34. The output of the first multiplier 36 provides a calculated quadrature signal 40 that can be sent to the motor drive control circuitry 28.
The angular rate component can be extracted by inputting the complex filtered second signal 32 to a second multiplier 42. The second multiplier 42 demodulates the complex filtered second signal 32 by multiplying the filtered signal 32 by a phase shifted signal 44 that is ninety-degrees from the reference signal 38. The phase-shifted signal 44 is derived by coupling the reference signal 38 to a ninety-degree phase shifter 46. The output of the second multiplier 42 provides the reported angular rate signal 22 indicative of the rotational rate externally induced to the gyroscope element 20. A low pass filter 48 may be used to remove any further signal components with a high frequency.
This type of system, however, has limitations. For example, the system requires a very precise narrow bandpass filter. The use of a narrow bandpass filter passes only the signal components within an expected range of frequencies. Using a narrow bandpass filter requires that the windows be delayed to match the delay of the signal induced by the filter. If a band rejection filter is be used, then the quadrature and the rate signals may contain elements of the noise and the final values are susceptible to DC offsets of the signal. Moreover, the system does not account for variations from device to device that may affect the relationship of the angular rate.
A need exists for an improved system for extracting the angular rate component from the output signal of a gyroscope sensor. It is, therefore, desirable to provide an improved procedure and apparatus for extracting signals to overcome most, if not all, of the preceding problems.